1. Field of the Invention.
This invention relates generally to a noise reduction circuit and, more particularly, to the noise reduction circuit for a video signal having a field memory.
2. Description of the related art.
In conventional magnetic recording/reproducing apparatus for recording and reproducing video signals, noise reduction circuits are used to decrease noise components included in the video signals. These noise reduction circuits are categorized into two types, i.e., one using the frame correlation and the other using the field correlation.
For example, a noise circuit using the frame correlation compares a pixel in the current frame with the identical pixel of the immediately preceding frame in a direction of the same time base. When the difference in level between the pixels upon comparison is relatively large, a corresponding picture is considered a motion picture. As a result, a video signal of the pixel in current frame is directly output. In this case, the motion picture corresponds to a video signal changing between the frames.
On the contrary, when the difference in level between the pixels is relatively small, a corresponding picture is considered a motionless picture (corresponds to a video signal which does not change between the frames). The difference is basically considered a noise component. This noise component is subtracted from the video signal of the current pixel, and the resultant signal is output, thereby reducing noise in the video signal.
Similar to the above circuit using the frame correlation, a noise reduction circuit using the field correlation compares a pixel in the current frame with the identical pixel of the immediately preceding field in a direction of the same time basis. In response to the difference upon comparison, the circuit directly outputs the video signal of the current pixel, or outputs a video signal obtained by subtracting a noise component corresponding to the difference from the video signal of the current pixel.
The noise reduction circuit using the field correlation can reduce the capacity of a field memory to 1/2 that of the type using the frame correlation. In addition, an after-image caused when the motion of a picture is rapidly changed can be reduced compared with the type using the frame correlation.
However, in the type using the field correlation, since the video signal is scanned by the interlace scanning method, although the pixel positions between fields coincide with each other in the horizontal direction, those in the vertical direction are shifted from each other by 1/2h (h=scanning line interval). For this reason, the vertical resolution is degraded.